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Post-employment benefits for associates (Details 7) - Other post-employment benefit plans
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]      
Healthcare cost trend rate assumed for next year 6.30% 6.50% 7.00%
Rate to which the cost trend rate is assumed to decline 4.50% 4.50% 4.50%
Year that the rate reaches the ultimate trend rate 2028 2028 2028